1. Field of the Invention
The present invention relates to an image forming apparatus, a control method therefor, and a computer program.
2. Description of the Related Art
An electrophotographic method is known as an image printing method used in a color image forming apparatus such as a color printer or color copying machine. According to the electrophotographic method, a latent image is formed on a photosensitive drum using a laser beam, and developed with a charged printing material (to be referred to as toner hereinafter). The image is printed by transferring the developed toner image onto transfer paper and fixing it.
These days, tandem type color image forming apparatuses are becoming popular. To increase the image forming speed of the electrophotographic color image forming apparatus, the tandem type color image forming apparatus comprises developing units and photosensitive drums equal in number to toner colors and sequentially transfers images of different colors onto an image conveyance belt or printing medium. It is known that the tandem type color image forming apparatus has a plurality of factors which cause a registration error. A variety of measures against the respective factors have been proposed.
These factors include the unevenness and attaching positional error of the lens of a deflecting scanning device, and the mounting positional error of the deflecting scanning device to the color image forming apparatus main body. Owing to these positional errors, the scan line inclines or skews, and the degree of inclination or skew (to be referred to as the profile hereinafter) is different between colors, causing a registration error.
The profile has different characteristics for respective image forming apparatuses, that is, printing engines, and for respective colors. FIGS. 22A to 22D show examples of the profile. In FIGS. 22A to 22D, the abscissa axis represents a position in the main scanning direction in the image forming apparatus. Lines 2201, 2202, 2203, and 2204 expressed as straight lines in the main scanning direction represent ideal characteristics free from a skew. Curves 2205, 2206, 2207, and 2208 represent the profiles of respective colors. More specifically, the curve 2205 represents a cyan (to be referred to as C hereafter) characteristic, the curve 2206 represents a magenta (to be referred to as M hereafter) characteristic, the curve 2207 represents a yellow (to be referred to as Y hereafter) characteristic, and the curve 2208 represents a black (to be referred to as K hereafter) characteristic. The ordinate axis represents a shift amount in the sub-scanning direction from an ideal characteristic. As is apparent from FIGS. 22A to 22D, the change point of the curve is different between colors. This difference appears as the registration error in image data after fixing.
As a measure against the registration error, there is proposed a method of measuring the degree of skew of a scan line using an optical sensor in the process of assembling a deflecting scanning device, mechanically rotating the lens to adjust the skew of the scan line, and fixing the lens with an adhesive (see Japanese Patent Laid-Open No. 2002-116394).
There is proposed a method of measuring the degree of inclination of a scan line using an optical sensor in the process of mounting a deflecting scanning device into a color image forming apparatus main body, mechanically inclining the deflecting scanning device to adjust the inclination of the scan line, and then mounting the deflecting scanning device into the apparatus main body (see Japanese Patent Laid-Open No. 2003-241131).
There is also proposed a method of measuring the degrees of inclination and skew of a scan line using an optical sensor, compensating bitmap image data to cancel them, and forming the compensated image (see Japanese Patent Laid-Open No. 2004-170755). This method electrically compensates the registration error by processing image data, and thus does not require a mechanical adjustment member or adjustment step in assembly. This method can downsize a color image forming apparatus, and deal with a registration error at a lower cost than those by the two mechanical adjustment methods described above. The electrical registration error compensation is divided into compensation of one pixel and that of less than one pixel.
In compensation of one pixel, pixels are offset one by one in the sub-scanning direction in accordance with the inclination and skew compensation amounts, as shown in FIGS. 23A to 23C. In the following description, a pixel position where the pixel is offset will be called a “scan line changing point”. In FIG. 23A, P1 to P5 are scan line changing points.
In order to reproduce data of the nth line, as shown in FIG. 23B, a coordinate conversion process is done for each pixel by offset. FIG. 23C shows an exposure image obtained by exposing the image carrier in accordance with image data having undergone color misregistration compensation for each pixel.
In compensation of less than one pixel, the tone value of bitmap image data is adjusted by preceding and succeeding pixels in the sub-scanning direction, as shown in FIGS. 24A to 24E.
FIG. 24A shows a main scan line having a positive inclination. FIG. 24A shows a case where the scan line is shifted by one pixel in the sub-scanning direction every time it proceeds by five pixels in the main scanning direction. FIG. 24B shows bitmap image of a horizontal straight line before performing density conversion. FIG. 24C shows a compensated bitmap image when performing compensation to cancel color misregistration caused by the inclination of a main scan line in FIG. 24A. To obtain such an image, the exposure amount of preceding and succeeding dots in the sub-scanning direction needs to be adjusted. FIG. 24D shows a bitmap image having undergone density conversion for adjusting the exposure ratio of preceding and succeeding pixels in the sub-scanning direction. FIG. 24E shows the exposure image of the bitmap image having undergone density conversion on the image carrier. The inclination of the main scan line is canceled to form a horizontal straight line.
That is, when the profile characteristic skews upward, as shown in FIG. 23A, bitmap image data before tone compensation is processed in a direction opposite to one indicated by the profile in the sub-scanning direction. By executing compensation of less than one pixel according to this method, an unnatural step generated by compensation of one pixel at a scan line changing point can be canceled to smooth the image.